Referring to FIG. 1, a schematic diagram of an upper bound and a lower bound is shown. As efficiency of notebook increases, processor also increases its power consumption. The processor of conventional notebook determines whether to start or cancel a throttling mechanism according to an upper bound UB and a lower bound LB. When power consumption is greater than the upper bound UB, the processor correspondingly starts the throttling mechanism to avoid residual capacity of battery being inadequate. Conversely, when power consumption is greater than the lower bound LB, the processor correspondingly cancels the throttling mechanism to increase efficiency of notebook.